Conventional active optical range sensors have been known to illuminate an object with light from a projecting light source such as an LED (Light Emitting Diode) and detect the light reflected from the object with a photodetector, so as to output a signal corresponding to the distance to the object. While PSD (Position Sensitive Detector) and the like have been known as optical range sensors of optical triangulation type which can simply measure the distance to the object, there have recently been expectations for development of optical range sensors of optical TOF (Time-Of-Flight) type in order to measure the distance more accurately.
Image sensors which can acquire range information and image information at the same time by the same chip have been in demand for automobiles, automated manufacturing systems in factories, and the like. There have been expectations for image sensors which acquire a range image constituted by a single or plurality of range information items separately from the image information. The TOF method is preferably used for such a range sensor.
Known as an example of range image sensors based on the TOF method is a solid-state imaging device disclosed in Patent Literature 1. The range image sensor disclosed in Patent Literature 1 is constituted by two-dimensionally arranged pixels each comprising a rectangular charge generation region; transfer gate electrodes respectively disposed along a set of opposing two sides of the charge generation region; floating drain regions for accumulating signal charges transferred by the transfer gate electrodes; exhausting gate electrodes, respectively disposed along a set of other opposing two sides of the charge generation region, for emitting background charges from the charge generation region; and exhausting drain regions for respectively receiving the background charges emitted by the exhausting gate electrodes. This range image sensor calculates the distance to the object according to the amounts of charges distributed into the floating drain regions. Since the background charges generated in the charge generation region are exhausted, the influence of background light is reduced, whereby the dynamic range improves.
Range image sensors such as those mentioned above have been desired to attain higher sensitivity, i.e., better signal to noise (S/N) ratio. Increasing the areal ratio of the charge generation region (hereinafter referred to as aperture ratio) is one of measures for attaining higher sensitivity.